In filter apparatus of the type disclosed in U.S. Pat. Nos. 4,090,965 and 4,639,315, web type filter material such as a filter cloth is mounted on a rotatable filter frame to form one or more enclosed filter effluent compartments on the rotatable frame. During filtering, liquid influent containing suspended solids is passed into an influent compartment and flows in a filter direction through the filter material into the filter effluent compartment for discharge to an outlet. Intermittent backwashing of the filter material is effected by moving the filter material past a suction head positioned to engage an outer side of the filter material and which is operative to draw filtered liquid from the filter effluent compartment through the filter material in a direction opposite the filter direction, to remove and discharge the entrapped solids with the backwash water. In general, a backwash step is initiated either by a timer at periodic intervals or by sensing a preselected rise in liquid level in the tank as an indication that the filter web is becoming clogged.
There are some solids which cannot be removed from the filter material by backwashing alone and solids continue to build up in the filter material and shorten the time interval between successive backwashings. The above patents also disclose a high pressure spray wash for washing the filter material after a number of backwash operations. The high pressure spray wash includes spray nozzles disposed either inside or outside the filter effluent compartments and arranged to direct high pressure liquid spray at one side of the filter material. The high pressure spray wash has been found effective to remove some of the solids that are not removed in the backwashing steps and thus increase the operational life of the filter material. The high pressure spray wash cannot be effected while the area of the filter material being washed is submerged in liquid and it is necessary to interrupt filtering during the high pressure wash. In the prior filter apparatus of this type, the high pressure wash operation was initiated either by a timer, based on an estimate of the appropriate time interval between high pressure washes, or manually at a time selected by a filter operator. However, the rate of influent flow to the filter and the composition of the influent varies over a wide range with time in the same installation and it has not been possible to accurately estimate the time at which the filter material should be subjected to a high pressure wash operation.